1. Field of the Invention
The present invention relates to a navigation device, and more particularly relates to a method for displaying a mark of a vehicle position on a road map.
2. Description of the Prior Art
A navigation system which searches for a route to a destination and provides guidance of the searched route is known for use in a vehicle and a portable terminal. The navigation device carries out route guidance and traffic information guidance by detecting a location of a vehicle, and showing the vehicle position on a road map shown on a display. Moreover, the navigation device searches for an optimal route to a destination, shows the searched route on the display, and, at the same time, shows an estimated arrival time at the destination along the searched route.
The navigation device employs, as means for acquiring a current vehicle position, dead reckoning which determines the vehicle position by using a distance sensor and a direction sensor on the vehicle, and a method for detecting the vehicle position according to GPS positioning using satellites.
The distance sensor obtains a travel distance according to a pulse signal generated every predetermined distance the vehicle travels. The direction sensor detects an acceleration in the lateral direction by means of a gyro when a vehicle starts, stops, or turns at a curve. Though dead reckoning has an advantage of detecting the vehicle position with a simple configuration, an error is accumulated as the travel distance increases, resulting in a decrease in accuracy. Although GPS positioning can obtain the absolute position of the vehicle based on the principles of trigonometry, the positioning involves difficulty when conditions of the radio waves degrade due to obstacles such as undulated terrains, tunnels, and tall buildings.
In order to compensate for the above shortcomings, the existing navigation device employs a hybrid method which uses both dead reckoning and GPS positioning. However, even with the hybrid method, there still exists an error between the actual vehicle position and the detected vehicle position. Therefore, the navigation device uses map data, in order to align the detected vehicle position on a road in the map data and show a vehicle position mark at the aligned position.
Methods for matching the vehicle position to a road in the map data include pattern matching and projection matching. The former compares a pattern of a travel trajectory of the detected vehicle position and patterns of roads with each other, thereby selecting an optimal road, and showing the vehicle position mark on the selected road. The latter selects an optimal road based on a distance and a direction of a road with respect to the detected vehicle position, thereby showing the vehicle position mark on the selected road.
Japanese Patent Laid-Open Publication No. 2001-041754 describes a technology which uses high-precision polygon data for showing roads and intersections in an urban map, and carries out matching with the polygon data thereby improving on a deviation in display of a vehicle position mark.
Projection matching, which selects a road candidate mainly according to the conditions of the distance and the direction, does not always provide correct road selection. For example, on a branch at a small angle in which a main road and a branch road intersect at an acute angle less than 90 degrees, even if a vehicle is traveling on the main road, the vehicle position mark may temporarily be shown on the branch road. Conversely, even when the vehicle is traveling on the branch road, the vehicle position mark may temporarily be shown on the main road.
For example, as shown in FIG. 7(A), it is assumed that a main road 200 and a branch road 201 intersect at a small angle, and the vehicle travels on the main road 200. N0, N1, . . . , N7 denote detected vehicle positions, and P0, P1, . . . , P7 denote matched positions obtained by the projection matching of the detected vehicle positions N0, N1, . . . , N7 to the road, namely, positions at which the vehicle position mark is shown. For the detected vehicle positions N0, N1, and N2, the matched positions P0, P1, and P2 are correctly shown on the main road 200. However, when the vehicle reaches the branch road 201, the matched positions P3, P4, and P5 of the detected vehicle positions N3, N4, and N5 are temporarily matched to the branch road 201, and then the matched positions P6 and P7 of the detected vehicle positions N6 and N7 are again matched to the main road 200.
Moreover, as shown in FIG. 7(B), when the vehicle actually travels from the main road 200 to the branch road 201, matched positions P3 and P4 of detected vehicle positions N3 and N4 are matched to the branch road 201, then matched positions P5 and P6 of detected vehicle positions N5 and N6 are temporarily matched to the main road 200, and finally, matched position P7 of detected vehicle position N7 is matched to the branch road 201.